Presented at the 3rd Annual Vegetable Crops Field Day, February 4,
1998

Good soil fertility management is one key ingredient for successful production
of melons in Arizona. Inadequate nutrients can lead to loss of plant vigor, susceptibility
to pests and, of course, lower marketable yields. On the other hand, excessive
applications of nutrients can lead to unnecessary production costs and, in the
case of nitrogen, pollution of groundwater supplies. Experience, soil testing,
plant tissue testing, and a knowledge of irrigation water quality are important
tools for proper fertility management for melons.

Nitrogen (N) and phosphorus (P) are the nutrients that have the greatest
influence on melon yields in Arizona. Without fertilization, Arizona soils
can rarely supply sufficient N or P for high-yielding crops. However, most
Arizona soils will supply adequate potassium (K) for melon yields. Under
some conditions, K deficiencies may occur. Such conditions include melons
after alfalfa, on very sandy soils, or early-planted fall melons. Deficiencies
of other plant nutrients are very rare for melons in Arizona. A well fertilized
cantaloupe crop yielding 20-30 tons/acre will take up about 90 lb N/acre,
25 lb P/acre, and 150 lb K/acre. A watermelon crop yielding 30-40 tons/acre
will take up about 100 lb N/acre, 30 lb P/acre, and 160 lb K/acre.

Because adequate N fertility is critical for good melon yields, it is
important to account for all sources of N available to the crop. These sources
include nitrate (NO3-N) in the soil at the beginning of the season and NO3-N
added in irrigation water. Soil NO3-N is measured in preplant soil samples.
Nitrate added in irrigation water can be determined by the formula: ppm
NO3-N in irrigation water * 2.7 = lbs. NO3-N/acre foot. For example, irrigation
water containing 10 ppm NO3-N will supply 27 lbs N per acre foot of water
applied.

Soil samples collected before planting can give important information for early-season
fertility decisions. Soil samples should be collected randomly to a 1
foot depth throughout "uniform" areas. The areas should be relatively
uniform with respect to soil texture and crop and soil management history.
Within each uniform area, a minimum of 20 samples should be collected,
then mixed together to form one sample. The samples can be analyzed for
a wide range of values, but a recommended minimum set of measurements
is: nitrate-N, available P and K, and pH, soluble salts and exchangeable
sodium percentage. Refer to Table 1 for the recommended
preplant fertilizer rates for melons.

Nitrogen is easily lost from soils because of nitrate leaching and volatilization
of gases. Therefore, it is recommended that N be split into two or more
applications during the season. These applications should be guided by
feedback from leaf petiole samples collected at various growth stages,
beginning at the 3-4 leaf stage. The leaf sampled should be the youngest
mature leaf. After runners develop, this is the 4th or 5th leaf from the
growing tip for cantaloupe, and the 5th or 6th leaf from the growing tip
for watermelon. Collect petioles from a minimum of 15-20 average plants
within uniform areas. Cool or dry the petioles immediately and submit
them to a laboratory for analysis. Refer to Figures 1
and 2 and Tables 2 and 3
for interpretation of cantaloupe and watermelon petiole NO3-N analysis.

Any N fertilizer source is usually suitable for use on melons. However,
if petiole NO3-N is in the "warning" or "deficient"
ranges, apply nitrate or urea sources. Also, use care when applying ammonium-N
sources at high rates or in sandy soils, because ammonia toxicity can result.

Issued in furtherance of Cooperative Extension work, acts
of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture,
James A. Christenson, Director Cooperative Extension, College of Agriculture
and Life Sciences, The University of Arizona.

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Information provided by Thomas L. Thompson, thompson@ag.arizona.edu
Associate Professor and Soils Specialist, College of Agriculture, The University of
Arizona.
Material written February 1998.